At what low outside temperature will a traditional straight air conditioning be damanged.

johncharles

This is sort of a food for though general question but I've thought about it more than I likely should.

Especially during the shoulder season we've had plenty of days that were warm during the day but quickly cool into the evening. Yesterday for example was around 80 but between about 5:30 and 7:30 a cold front came through and the temp dropped to 53 in just an hour or so.

During that time the house was still pretty warm and the AC ran at least once while it was pretty cold out at least in the low 60's before I realized how cold it was.

With that said this has happened many many times typically in the evening in Sep and May and nothing ever seems wrong or damaged.

Yet if you visit some parts of the internet they would make it sound like running the AC when it's that cold out will instantly kill your unit which as I said doesn't seem accurate at all.

I guess my question is is this really true, or is this just some FUD (Fear Uncertainty and Doubt) that can be mostly ignored.

Jamie Hall

Um... 60 is cold? It might get up to 60 here today, in the sun, but I doubt it...

In the meantime, the heat pump is quite happy. In fact, it is happy down around 0.

So... * don't think you have anything to worry about. As you say, FUD.

pecmsg

“Starting” an A/C when it’s been sitting at 60* is un advisable. If it’s already running then the oil is warm and things have stabilized. 

johncharles

Can you clarify startup? Are we talking about just the start between cycles or like it's been off for 3 days and we start it?

Let's say it's running about 15 minutes every 45 or so minutes, typical of a moderate day let's say 76-80* outside.

It sounds like if it starts running when its 70 and then the temp drops during that 15 minute period that's fine. What about if it's been off for about 40 or so minutes and then it starts, and between the current cycle and the last cycle it has dropped down to let's 58?

Second and I might be way over thinking this but even if the oil is more viscous wouldn't starting the compressor with more viscous oil just cause it to warm up faster and hence warm up the oil?

At, let's say, 55 is the oil really already super thick sludge? And if the unit ran about 45 minutes ago, are the core components of the compressor even actually anywhere close to 55* anyway?

ChrisJ

There's two issues.

1: refrigerant migration. Refrigerant tends to lay in the coldest part of the system. If the system has been running the compressor will be warm and the evaporator will likely be quite cool. However, if the compressor has been off for a while and even worse, a furnace has been running warming the evaporator, a lot of refrigerant may be laying in the compressor and the section line going to it. This isn't going to be good for the compressor when it starts. It dilutes the compressor oil and, may also cause the compressor to pump liquid. This situation isn't great. A crank case heater will help.


2: Low ambient temperatures cause low head pressure. This sounds like a good thing, but it can starve the evaporator of refrigerant which could cause icing problems etc. As head pressure drops the metering device is expected to open further to compensate, but they can only open so much. From what I recall, most are sized to run down to about 55-60F in this regard. Assuming the system has been active to avoid the previous problem.


@pecmsg Something I've been wondering about. How do systems deal with refrigerant laying in the suction line under such conditions? Apparently there are tons and tons of cool only systems in commercial that only use CCH and a fan controller? Even if they're using a head master, isn't refrigerant collecting in the suction line an issue?

Matt_67

It’s more about refrigerant diluting the oil than the viscosity. Over time refrigerant will migrate to the coldest part of the system, so during cold weather that might be the compressor. Units designed to run in colder weather generally have a crankcase heater to prevent that from happening. Oil and liquid refrigerant is a poor lubricant and will eventually damage the compressor. It’s probably not that big a deal if it happens occasionally.

Jamie Hall

Humph. Reading all of the above leads me to conclude that the engineering of window shakers is much worse than I had thought, both in terms of construction and in terms of materials choices. The automotive industry seems to have done much better -- the climate control system in most cars for the last 30 years at least has no difficulty at all in starting up in subfreezing temperatures, and, folks, the air conditioning compressor runs (or should run) any time the defrost mode is selected (if it doesn't, odds are it's the controller or the clutch).

For a definitive answer, I guess one should check the product literature -- it should say, somewhere, what the operating and storage ranges are.

ChrisJ

Jamie Hall said:

Humph. Reading all of the above leads me to conclude that the engineering of window shakers is much worse than I had thought, both in terms of construction and in terms of materials choices. The automotive industry seems to have done much better -- the climate control system in most cars for the last 30 years at least has no difficulty at all in starting up in subfreezing temperatures, and, folks, the air conditioning compressor runs (or should run) any time the defrost mode is selected (if it doesn't, odds are it's the controller or the clutch).

For a definitive answer, I guess one should check the product literature -- it should say, somewhere, what the operating and storage ranges are.

Jamie,
Automotive systems aren't hermetic they're belt driven. This eliminates the main issue described above. There literally is no crank case full of oil with a motor in it.

They also often have accumulators and or receivers and they can cycle the compressor easily if need be. The compressors are also often variable displacement now as well.

Window air conditioners are much much worse than they need to be. They're made as cheap as possible, period.

pecmsg

ChrisJ said:

There's two issues. 1: refrigerant migration. Refrigerant tends to lay in the coldest part of the system. If the system has been running the compressor will be warm and the evaporator will likely be quite cool. However, if the compressor has been off for a while and even worse, a furnace has been running warming the evaporator, a lot of refrigerant may be laying in the compressor and the section line going to it. This isn't going to be good for the compressor when it starts. It dilutes the compressor oil and, may also cause the compressor to pump liquid. This situation isn't great. A crank case heater will help. 2: Low ambient temperatures cause low head pressure. This sounds like a good thing, but it can starve the evaporator of refrigerant which could cause icing problems etc. As head pressure drops the metering device is expected to open further to compensate, but they can only open so much. From what I recall, most are sized to run down to about 55-60F in this regard. Assuming the system has been active to avoid the previous problem. @pecmsg Something I've been wondering about. How do systems deal with refrigerant laying in the suction line under such conditions? Apparently there are tons and tons of cool only systems in commercial that only use CCH and a fan controller? Even if they're using a head master, isn't refrigerant collecting in the suction line an issue?

Crankcase heaters prevent refrigerant from migrating during the off cycle. They don’t help with slugs of liquid. 

In refrigeration or where line sets are buried a pump down system is preferred. 

ChrisJ

pecmsg said:

ChrisJ said:

There's two issues.

1: refrigerant migration. Refrigerant tends to lay in the coldest part of the system. If the system has been running the compressor will be warm and the evaporator will likely be quite cool. However, if the compressor has been off for a while and even worse, a furnace has been running warming the evaporator, a lot of refrigerant may be laying in the compressor and the section line going to it. This isn't going to be good for the compressor when it starts. It dilutes the compressor oil and, may also cause the compressor to pump liquid. This situation isn't great. A crank case heater will help.


2: Low ambient temperatures cause low head pressure. This sounds like a good thing, but it can starve the evaporator of refrigerant which could cause icing problems etc. As head pressure drops the metering device is expected to open further to compensate, but they can only open so much. From what I recall, most are sized to run down to about 55-60F in this regard. Assuming the system has been active to avoid the previous problem.


@pecmsg Something I've been wondering about. How do systems deal with refrigerant laying in the suction line under such conditions? Apparently there are tons and tons of cool only systems in commercial that only use CCH and a fan controller? Even if they're using a head master, isn't refrigerant collecting in the suction line an issue?

Crankcase heaters prevent refrigerant from migrating during the off cycle. They don’t help with slugs of liquid. 

In refrigeration or where line sets are buried a pump down system is preferred. 

But what about systems where the lineset isn't buried, but is still exposed outside? Wouldn't refrigerant tend to condense in that cold area during the winter even if the CCH is in use? A few feet of 1 1/8" tubing can hold an awful lot of liquid.

pecmsg

ChrisJ said:

ChrisJ said:

There's two issues. 1: refrigerant migration. Refrigerant tends to lay in the coldest part of the system. If the system has been running the compressor will be warm and the evaporator will likely be quite cool. However, if the compressor has been off for a while and even worse, a furnace has been running warming the evaporator, a lot of refrigerant may be laying in the compressor and the section line going to it. This isn't going to be good for the compressor when it starts. It dilutes the compressor oil and, may also cause the compressor to pump liquid. This situation isn't great. A crank case heater will help. 2: Low ambient temperatures cause low head pressure. This sounds like a good thing, but it can starve the evaporator of refrigerant which could cause icing problems etc. As head pressure drops the metering device is expected to open further to compensate, but they can only open so much. From what I recall, most are sized to run down to about 55-60F in this regard. Assuming the system has been active to avoid the previous problem. @pecmsg Something I've been wondering about. How do systems deal with refrigerant laying in the suction line under such conditions? Apparently there are tons and tons of cool only systems in commercial that only use CCH and a fan controller? Even if they're using a head master, isn't refrigerant collecting in the suction line an issue?

Crankcase heaters prevent refrigerant from migrating during the off cycle. They don’t help with slugs of liquid. 

In refrigeration or where line sets are buried a pump down system is preferred. 

But what about systems where the lineset isn't buried, but is still exposed outside? Wouldn't refrigerant tend to condense in that cold area during the winter even if the CCH is in use? A few feet of 1 1/8" tubing can hold an awful lot of liquid.

Yes, quite a bit will condense in the suction line. 

ChrisJ

pecmsg said:

ChrisJ said:

pecmsg said:

ChrisJ said:

There's two issues.

1: refrigerant migration. Refrigerant tends to lay in the coldest part of the system. If the system has been running the compressor will be warm and the evaporator will likely be quite cool. However, if the compressor has been off for a while and even worse, a furnace has been running warming the evaporator, a lot of refrigerant may be laying in the compressor and the section line going to it. This isn't going to be good for the compressor when it starts. It dilutes the compressor oil and, may also cause the compressor to pump liquid. This situation isn't great. A crank case heater will help.


2: Low ambient temperatures cause low head pressure. This sounds like a good thing, but it can starve the evaporator of refrigerant which could cause icing problems etc. As head pressure drops the metering device is expected to open further to compensate, but they can only open so much. From what I recall, most are sized to run down to about 55-60F in this regard. Assuming the system has been active to avoid the previous problem.


@pecmsg Something I've been wondering about. How do systems deal with refrigerant laying in the suction line under such conditions? Apparently there are tons and tons of cool only systems in commercial that only use CCH and a fan controller? Even if they're using a head master, isn't refrigerant collecting in the suction line an issue?

Crankcase heaters prevent refrigerant from migrating during the off cycle. They don’t help with slugs of liquid. 

In refrigeration or where line sets are buried a pump down system is preferred. 

But what about systems where the lineset isn't buried, but is still exposed outside? Wouldn't refrigerant tend to condense in that cold area during the winter even if the CCH is in use? A few feet of 1 1/8" tubing can hold an awful lot of liquid.

Yes, quite a bit will condense in the suction line. 


And the only way to deal with that is a pump down system? Most server room setups etc do not have that, do they?

One thing I've been baffled by is when manufacturers say CCH is not required if the charge is below X amount. Why does the charge being below that amount all of a sudden change the game? I have to assume they have a very real reason for this, but I haven't figured it out.

Matt_67

Pump down is pretty common on the server room units I’ve worked on. We do quite a bit of work with low ambient cooling for industrial motor control centers and they almost all have pump down systems, sometimes suction accumulators and receiver heaters as well. They require full capacity cooling at temps down to -25. Most use headmaster valves for discharge pressure control but some use condenser fan speed controls as well.

johncharles

I think this went a little off topic from 55-60* outside temp on a conventional residential split AC system.

Reading through the posts above it looks like the issue is that happen at those lower temps are that the suction line gets filled with liquid and the compressor has to pump liquid when it starts which is not good.

Some follow ups, as soon as the compressor starts the pressure in the suction line starts to drop, and inevitably in very short order the former liquid will have started to vaporize. How long does that take to occur? I would imagine just a few seconds. How much harm occurs to the compressor in those few seconds, before the liquid becomes vapor?

Second, I've heard that scroll compressors have internal equalization valves which allow the pressure to equalize when the compressor is off. This is why you don't need a hard start with a TXV on a scroll but do need one on a reciprocating compressor. Would that mean that even in the summer a scroll compressor is always starting with liquid on both sides since the pressure equalization would have allowed that liquid to have passed back to the other side of the compressor?

I guess I'm understanding the general issues with starting an AC under those colder conditions say 55-60 degrees. But I'm trying to understand how serious these issues are? From the above it sounds very unlikely that you will totally kill your compressor starting it in 55* weather, but also that it might if done repeatedly stress it such that it might over a long run shorten its life. But by how much? If an AC system can last 30 years, will doing this make it last only 28 years (not a huge deal) or will it only survive 10 years a huge reduction in life expectancy?

clammy

Operating residential ac system is low ambient is not highly recommend by most manufactures and may void any warranty w prolonged operation . When a customer desires low ambient operation usually a crank case heater ,and a low ambient head pressure controller is added and a time delay relay aside from a hard start kit some may add a liquid line solonoid to help stop liquid migration on the off cycle . Ions ago units can w suction line accumulators which where a good thing now adays forget about it . I don’t think I would do a pump down unless the u it had a properly sized reciever good luck ,ICM makes a low ambient controller kit that works very well and has setting for either a sleeve bearings or ball bearing motors I ve used in the past w good results . Properly adding the correct controls for average low ambient cooling is not a huge leap or a big deal but if you want cooling below 40 outdoors temps don’t accept the equiptment to last forever under really low ambient sin which case get a unit that’s set up and designed for such ,I believe most Bosch inverter compressor would fitting the bill and little better then creating your own Frankenstein condenser ,anything can be done it s a matter of money time and are you that smarter then then the manufactures ?
Peace and good luck clammy

ChrisJ

clammy said:

Operating residential ac system is low ambient is not highly recommend by most manufactures and may void any warranty w prolonged operation . When a customer desires low ambient operation usually a crank case heater ,and a low ambient head pressure controller is added and a time delay relay aside from a hard start kit some may add a liquid line solonoid to help stop liquid migration on the off cycle . Ions ago units can w suction line accumulators which where a good thing now adays forget about it . I don’t think I would do a pump down unless the u it had a properly sized reciever good luck ,ICM makes a low ambient controller kit that works very well and has setting for either a sleeve bearings or ball bearing motors I ve used in the past w good results . Properly adding the correct controls for average low ambient cooling is not a huge leap or a big deal but if you want cooling below 40 outdoors temps don’t accept the equiptment to last forever under really low ambient sin which case get a unit that’s set up and designed for such ,I believe most Bosch inverter compressor would fitting the bill and little better then creating your own Frankenstein condenser ,anything can be done it s a matter of money time and are you that smarter then then the manufactures ?
Peace and good luck clammy

I added the ICM controller to mine, as well as swapped out the OEM fan motor for the highest temp rated AO Smith ball bearing motor I could find. I also added a CCH which I just had to replace this year as I found the one I installed in 2017 was open. Oh, and I installed a freeze stat at the evaporator.

In my case, my system runs a good amount when it's in the 50s out, and sometimes when it's in the 40s but not as much. It's kinda cool to see a clear sight glass with 410A when it's cool outside.

jumper

Good explanations above. Why does refrigeration machinery have low pressure safety?

clammy

Refrigeration equipment uses a low pressure safety switches to prevent the compressor from operating below it design which might result in slugging of refrigerate and oil also so it does not operate without refrigerant like a leak but usually it used in a pump down system to cycle the compressor on low pressure when the liquid line solenoid closes I response to a box reaching temperature and it’s thermostat opening the circuit .pretty basic stuff not used in most modern air conditioning but 40 to 50 years ago it was very common to see in in water cooled condensers and even in ancient roof tops and splits and all that stuff lasted decades now it 15 to 20 and off to the recycler then China . Really funny thing is 30 years ago most all residential ac yuck had a high and low pressure switch some where smart and had a resettable hi pressure duh it trips guess the unit s pig dirty ,boy have times and stuff changed but I’m not for the better or the better for who$
Peace and good luck clammy